This invention relates to an integrator using the switched current technique.
Switched current circuits are disclosed in a book entitled "Switched-Currents an analogue technique for digital technology" edited by C. Toumazou, J. B. Hughes and N. C. Battersby published by Peter Peregrinus, ISBN 0 86341 294 7.
First order filter building blocks which perform the bilinear z-transform have well known advantages. Unlike Euler integrators, they have no excess phase and so map to the z-plane with guaranteed stability giving the resulting filter a distortion free amplitude response, even in filters with clock frequencies approaching the Nyquist frequency. This makes the bilinear integrator especially suitable for high frequency filters.
In the past the difficulty of making switched capacitor bilinear z-transform integrators has led to the use of LDI techniques to produce biquadratic sections with bilinear mapping. While a similar approach is adopted for active ladder filters the simulation of the terminations is only approximate. Consequently, an integrator/summer which can perform true bilinear z-transformation is highly desirable.
Several switched current bilinear z-transform integrators have been proposed; for example that shown in the book edited by Toumazou, Hughes and Battersby at page 48. An alternative bilinear z-transform switched current integrator is disclosed in a paper by I. Song and G. W. Roberts entitled "A fifth order bilinear switched current Chebyshev filter" which was presented at the IEEE International Symposium on Circuits and Systems in 1993 and published in the Conference Proceedings at pages 1097 to 1100.
The common factor of all switched current bilinear z-transform integrators previously known to the inventors is that they operate with only one sample per clock period. In addition, all except the Song and Roberts integrator are single ended circuits and require current mirrors to produce signal inversion. Unfortunately these, mirrors introduce excess phase errors and to keep these errors small the clock frequency must be made large compared with the filter cut off frequency, thus nullifying one of the major advantages of bilinear mapping.